Seismologists register the intensity of an earthquake in units known as moment magnitude, which measure how much energy was released when the rocks along a fault moved during the quake.
The moment magnitude scale, as it is known, replaced one developed by an American seismologist, Charles Richter, that was used until the 1970s. The Richter scale was found to be inaccurate for very large earthquakes.
The moment magnitude scale can be hard for nonexperts to decipher. It is logarithmic, meaning that each whole number of magnitude represents about a 32-fold increase in the amount of energy released during a rupture.
So, for instance, an earthquake with a magnitude of 2.0 is not twice as strong as a quake with a magnitude of 1.0. Instead, the amplitude of shaking would be 10 times as great, and it would release 32 times as much energy.
The strongest earthquake ever recorded, a 9.5-magnitude one that occurred in Chile in 1960, was 30,000 times more powerful than a 6.5-magnitude quake, which itself can be very destructive.
Magnitude is far from a complete measure of a specific quake’s destructiveness, however. Other geological factors, like the location and depth of the fault and the type of rocks and soil it occurs in, can affect the amount of shaking and destruction that an earthquake can cause.
Construction methods and building standards can also play a major role in the amount of damage and the number of casualties. The timing of the event and whether people are at home, at work or out and about are also factors.
Scientists calculate a quake’s moment magnitude by using data from a network of instruments, called seismometers, spread across the world. Those instruments record the waves of shaking from the quake’s point of origin moving through the earth.
Worldwide, the frequency of earthquakes has remained largely unchanged over many decades of study. On average, there are about 1,500 quakes of a magnitude of 5.0 or higher every year. Of these, about 15 have a magnitude of 7.0 or higher.
People nearby might not feel a tremor of less than 4.0 magnitude. A 5.0 event might rattle nerves but do only light damage. At 6.0, moderate damage can be expected, especially to older and less resilient structures.
Strong quakes of 7.0 and up can do major damage over a wide area, and a quake of 8.0 or greater anywhere near a population center would probably cause catastrophic damage and loss of life.
Theoretically, a quake of magnitude 10 is possible, but the fault would have to be about 8,000 miles long, or about one-third of Earth’s circumference.
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